gpio: correctly use const char * const

On my previous patch I was overly hasty and made the suffixes string
array
const char const *suffixes, instaed of const char * const suffixes. This
patch corrects that

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>